body {
    font: 24px Helvetica;
    background: url("../img/bc.png") no-repeat center center fixed;
    -webkit-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}

.dropbtn {
    background-color: #4CAF50;
    color: white;
    padding: 16px;
    font-size: 16px;
    border: none;
    cursor: pointer;
}

.dropdown {
    position: relative;
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: #f9f9f9;
    min-width: 160px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
}

.dropdown-content a {
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
    text-align: center;
    font-size: 15px;
}

.dropdown-content a:hover {background-color: #f1f1f1}

.dropdown:hover .dropdown-content {
    display: block;
}

.dropdown:hover .dropbtn {
    background-color: #3e8e41;
}
.main > article ul {
    list-style: none;

}

article li {
    display: inline;
}


.main {
    min-height: 500px;
    margin: 0px;
    padding: 0px;
    display: flex; /*设置该div为一个弹性盒容器*/
    flex-flow: row; /*子元素按横轴方向顺序排列*/
}

.main > article {
    border: 2px solid #FFBA41;
    margin: 2px;
    padding: 15px;
    padding-left: 20px;
    border-radius: 7pt; /*pt也是文字大小的一种单位，1pt=px*3/4 */
    width: 100%;
    height: auto;
}

header, footer {
    display: block;
    margin: 4px;
    padding: 5px;
    min-height: 100px;
    border: 2px solid #FFBA41;
    border-radius: 7pt;
}

@media all and (max-width: 640px) {
    /*当屏幕小于640px时*/
    .main {
        flex-flow: column; /*弹性盒中的子元素按纵轴方向排列*/
    }

    .main > article {
        order: 0; /*将子元素都设置成同一个值，指按自然顺序排列*/
    }

    .main > nav, .main > aside footer {
        min-height: 50px;
        max-height: 50px;
    }

}